1. Field of the Invention
The present invention relates to a method for manufacturing a solid oxide film and a method for manufacturing a solid oxide fuel cell using the solid oxide film.
2. Related Art Statement
Recently, fuel cells have been recognized as a viable power generating source. The fuel cell is capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from the limitation of Carnot's cycle, the fuel cell essentially has a high energy conversion efficiency. Further, various fuels such as naphtha, natural gas, methanol, coal reformed gas and heavy oil may be used, additionally, these fuels may be used with a low pollutant level. Moreover, the power generating efficiency of fuel cells is not influenced by the scale of the equipment. Therefore, power generation with the aid of fuel cells is an extremely promising technique.
Particularly, since the solid oxide fuel cell (hereinafter abbreviated as SOFC) operates at a high temperature of 1000.degree. C. or more, the reaction of electrodes thereof is very active and the use of a noble metal catalyst such as expensive platinum is not required. In addition, since the SOFC has a low polarization and a relatively high output voltage, the energy conversion efficiency is considerably higher than that of other fuel cells. Furthermore, since the SOFC is constructed with solid materials, it is stable in structure and has a long service life.
In such SOFC, it is desired to make the solid oxide film as this as possible. However, in a chemical vapor deposition method or an electrochemical vapor deposition method, which are conventional methods for manufacturing the solid oxide thin film, there are drawbacks that an apparatus for manufacturing the solid oxide film becomes large, an area on which the solid oxide film can be formed is small and a processing speed for forming the film is slow. Therefore, the cost for manufacturing the film becomes high and it is difficult to obtain a large area solid oxide film. Additionally, in case of the electrochemical vapor deposition method, the shape of the substrate is limited to be cylindrical.
For instance, it has been known; (ref. Sunshine Journal 1981, Vol. 2, No. 1 pps 32-35) how to manufacture a solid oxide fuel cell with the aid of plasma spray coating; and the plasma spray coating is excellent in that a thin solid oxide film with a high density can be formed quickly in an easy manner.
Further, in Japanese Preliminarily Patent Publication Nos. 61-198569 and 61-195570, it is known that is a spray substance, in which cerium oxide or zirconium oxide and metal oxide such as alkaline-earth metals or rare-earth element are soluted, is sprayed on a substrate with the aid of the plasma spray coating after the particle size of the material is adjusted, it forms a solid oxide film on the substrate.
However, airtightness of the film formed by the plasma spray coating is generally low. Therefore, when a solid oxide film of the solid oxide fuel cell is formed by the plasma spray coating, the airtightness of the film is not sufficient. Therefore, when operating such SOFC, hydrogen, mono carbonate oxide, etc. is leaked through the solid oxide film. Then, the electromotive force per one SOFC becomes smaller than 1 V, and the output thereof is decreased, so that the energy converting efficiency from fuel to electric power is aggravated.
In this case, it is considered to make the solid oxide film thick to prevent the leakage of fuel, However, a diffused resistor for an ion diffusion in a bulk becomes so large that a resistor of the cell becomes large. Therefore, it is strongly desired to develop a technique to improve the power output of the unit SOFC by making the solid oxide film thin and making the density of the film high so that fuel leakage is not generated.